Concrete is typically delivered at a job site to remote areas via a pump that pumps the concrete through a series of boom-connected pipes terminating in a flexible delivery hose. The flow of concrete is typically controlled at a point remote from the delivery hose and thus even when the concrete flow is stopped at its source, it is not unusual for concrete to continue to flow or drip from the end of the delivery hose. This condition can be very undesirable at certain job sites.
A number of devices have been created to prevent the residual flow of concrete out the delivery hose. One such device is described in U.S. Pat. No. 6,871,832 by Lehnhardt, et al., as a flexible cylindrical sleeve disposed within a one-piece rigid casing that surrounds the delivery hose. When pressurized gas is forced into the space between the sleeve and casing, the sleeve then expands and constricts the hose to prevent the flow of concrete. In practice, the rigid casing is made from a long, rigid fiberglass tube with steel sealing caps on either side which adds significant weight to the device.
Another device is described in U.S. Pat. No. 6,968,861 by Judge as a substantially sealed flexible bladder in a casing where material flow through the discharge hose is prevented by introducing compressed air into the flexible bladder, causing the bladder to expand and constrict the discharge hose. In practice the casing is made from heavy stainless steel which causes the device to have significant weight.
In many cases, the flexible delivery hose is the last section on a truck-mounted articulating arm with the delivery pipes connected to the arms of the boom. With some of these boom trucks being able to deliver concrete to over 200 feet away from the truck, the combined weight of the arms, pipe and concrete can cause stability concerns. Because of this, boom truck manufacturers have strict weight limits on what equipment can be mounted to the pipes. The last section, where the flexible delivery hose is connected, is furthest out and has the greatest influence on stability and therefore the greatest restriction on the amount of weight that can be added to it. Unfortunately, this last section is where the relatively heavy flow control devices are added to the boom truck which lowers their stability safety factor.